Fluid-powered motors have been in use in drilling assemblies in the past. These designs are primarily a fixed stator rotating rotor, which are powered by fluid flow based on the original principles developed by Moineau. Typical of such single-rotor, progressive cavity downhole motor designs used in drilling are U.S. Pat. Nos. 4,711,006 and 4,397,619. The stator in Moineau motors is built out of elastic material like rubber. Other designs have put single-rotor downhole power sections in several components in series, with each stage using a rotor connected to the rotor of the next stage. Typical of these designs are U.S. Pat. Nos. 4,011,917 and 4,764,094.
Dual-rotor devices have been used as pumps. U.S. Pat. No. 4,820,135 uses a twin-rotor device which is fluid-operated which has output shafts connected to a downhole pump, which is also of the twin-rotor type, for use in producing low-pressure formations and especially if pumping three-phase media (gas-oil-sand). In essence, the twin-rotor design provides the mechanical energy to rotate another twin-rotor downhole pump to pump formation fluids and gases to the surface. U.S. Pat. No. 4,314,615 illustrates a self-propelled drilling head used in large-bore applications where hydraulic fluid is provided to drive twin-rotor motors through supply and return lines. The motors, through a complex planetary gear system, are connected to a bit. The technology and tools shown in U.S. Pat. No. 4,314,615 are used to drill mining shafts and tunnels.
Despite all these prior developments, what has been lacking is a compact design suitable in drilling a typical wellbore which has the desirable features of providing sufficient torque and power to the bit to accomplish the drilling in an expeditious manner. The disadvantages of the single-rotor designs is that they required complex controls to avoid damage if the bit became stuck or if the bit was suddenly picked up while fluid was circulating and the load on the bit relieved. Impurities in the mud were also a problem for the rubber of the stator in this design. Entrained solids and gas were particularly an issue in the reliable operation of the single-rotor, Moineau-type mud motors. Temperature limitations of the Moineau-type mud motor cause unreliable operation, especially for geothermal drilling applications. The control requirements, as well as the output limitations of the single-rotor designs, have been overcome by the present invention, which provides a compact design using a downhole motor having a twin-rotor design which is geared to the bit.
In directional drilling in the past, universal joints have been used, as indicated in some of the above-mentioned patents, to connect the output of the single-rotor power section to the drillbit. Universal joints have also been used to accommodate an offset in the motor housing or drillstring to permit directional drilling. One of the advantageous features of the design of the present invention is to provide, in a compact bottomhole assembly, an angular bend which is accomplished through the gearing of the output of the twin rotors to the drive for the bit. The gearing can be accomplished with a speed reduction using a straight cut gear meshing with an open structure comprising of spaced rods which will give long life in the hostile mud environment. Accordingly, complex structures that use universal joints are eliminated in the present design which can optionally provide for a bend angle as required and accomplish the connection between the bit drive and the rotating rotor through a gear system involving the requisite angular offset. By adaptation of a twin-rotor design used primarily in pumping applications, a compact downhole motor has been developed which can run on the circulating mud, with fewer controls, and can be constructed to accommodate directional drilling. Additionally, vibration is eliminated, which is common in Moineau motors due to orbital movements. Therefore, measurement while drilling procedures can be achieved much more accurately and economically with the present invention. Those and other beneficial features of the present invention will become apparent to those of ordinary skill in the art by a review of the specification and the drawings.